Electrophotographic marking is a well known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a substantially uniformly charged photoreceptor with a light image representation of a desired document. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image to form a toner image. That toner image is then transferred from the photoreceptor onto a copy substrate, such as a sheet of paper. The transferred toner image is then fused to the copy substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
One method of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is typically comprised of a laser light source (or sources) and a rotating polygon having a plurality of mirrored facets. The light source radiates a laser beam onto the polygon facets. The facets reflect the beam onto the photoreceptor, producing a light spot. As the polygon rotates the spot traces lines, referred to as scan lines, on the photoreceptor. By moving the photoreceptor in a process direction as the spot traces scan lines in the fast scan direction, the surface of the photoreceptor is raster scanned by the spot. During scanning, the laser beam is modulated with image information so as to produce a predetermined latent image on the photoreceptor.
While raster output scanners have been very successful, when printing at very high speeds significant problems must be overcome. For example, consider an electrophotographic marking machine printing at 120 pages per minute. This requires printing about 22 inches of paper per second. For various reasons (cost, noise, reliability) it is desirable that the polygon rotation should be limited to around 20,000 revolutions per minute or less. A dual laser diode ROS, which produces two scan lines at 600 Spots per inch at a time, having a 20 facet polygon rotating at about 19,800 revolutions per minute would meet the printing speed requirements. Unfortunately, such a ROS has problems. First, because of its large number of facets the ROS itself would be very large and would have a relatively long scan lens focal length. These characteristics are not compatible with compact, small footprint (desktop) printers. Additionally, such a large, multifaceted polygon would be rather expensive.
Therefore, a new raster output scanner assembly that is capable of printing at very high speeds and that is compatible with compact printers would be beneficial. Even more beneficial would be a new raster output scanner assembly that is capable of printing at very high speeds and that is suitable for use in a low cost, compact printer.